Turbomolecular Pump with a Flexible Mount

ABSTRACT

The flexibly-mounted turbomolecular pump comprises a leaktight casing mounted on a structure via a mechanical damper, a vertical support driven by an electric motor, radial magnetic bearings, an axial magnetic bearing, radial detectors, an axial detector, an emergency mechanical bearing, and a control module comprising power supply circuits and servo-control circuits for the axial and radial magnetic bearings. The servo-control circuits present low stiffness and include filter circuits for defining a limited passband. The control module comprises a comparator unit for detecting abnormally large movements of the rotor from signals delivered by the axial and radial detectors, which movements are greater than predetermined thresholds. The servo-control circuits include a restabilization network and a unit for selectively activating the restabilization network during a predetermined duration after a time at which the crossing of at least one of the predetermined thresholds is detected during a movement of the rotor.

FIELD OF THE INVENTION

The present invention relates to a turbomolecular pump with a flexible mount comprising a leaktight casing designed in such a manner as to be capable of being mounted on a structure via a mechanical damper, a vertical support secured to the casing and located therein for supporting a hollow rotor fitted with fins and driven in rotation by an electric motor, at least first and second radial magnetic bearings mounted on the support, at least one axial magnetic bearing mounted on the support, radial detectors for detecting the radial position of the rotor, at least one axial detector for detecting the axial position of the rotor, at least one emergency mechanical bearing, and a control module including at least circuits for powering the electric motor and the axial and radial magnetic bearings, and servo-control circuits for servo-controlling the axial and radial magnetic bearings suitable for correcting any movement of the rotor in translation along three axes xyz in an orthonormal frame of reference and any tilting relative to two tilt axes.

PRIOR ART

Turbomolecular pumps mounted using active magnetic bearings are already described, e.g. in patent documents FR 2 747 431 and WO 2005/038263.

In the majority of applications, such as semiconductor production, a turbomolecular pump on magnetic bearings is not exposed to external disturbances.

Under such circumstances, pumps are generally fastened rigidly on a support and they do not move. Position servo-control is then optimized to ensure that the pump can be mounted in any position and to ensure that the pump generates a minimum amount of noise and vibration.

Nevertheless, certain applications exist, e.g. such as electron microscopes, that require a level of vibration that is extremely low. In such applications requiring a low level of vibration, the casing of the pump is not rigidly mounted on a structure, rather the casing is decoupled from the structure by a mechanical damper, so as to define a flexible mount.

In certain applications implementing such a flexible mounting of the pump, it may be necessary for the user to change the position of or to move the tool-and-pump assembly while the pump is in rotation.

Setting servo-control for such applications becomes extremely difficult.

In order to obtain the specified low level of vibration, an application with a low level of vibration requires servo-control to present low stiffness and a limited passband.

Nevertheless, under such circumstances, any movement of the casing of the pump can cause the rotating shaft to come into contact with the emergency bearings, thereby generating noise, wear, and vibration.

After a movement of the rotor, initiated at an instant t₀ and making contact at an instant t_(i), with the rotor moving in at least one direction through a distance X_(m) corresponding to coming into contact with the rolling members of an emergency bearing, the behavior of the servo-control loop becomes very complex and non-linear, and there is a risk of permanent oscillation. FIG. 3 shows the behavior that follows the application of such an impact, with permanent oscillations 200 due to instability in the servo-control.

Patent document EP 1 045 821 A2 discloses a turbomolecular pump having a rotor mounted on magnetic bearings and a system for monitoring the currents flowing in the electromagnets of the magnetic bearings in order to switch off the power supply to the magnetic bearings permanently if the currents exceed a predetermined threshold during a predetermined length of time. The system serves to avoid non-reversible destabilization of a position servo-control loop as a result of making contact with the emergency bearings, however it does not enable wear of the parts to be reduced by acting in preventive manner, since that system is designed solely to switch off the electrical power supply, thereby necessarily leading to contact between the rotor and the emergency bearings.

OBJECT AND DEFINITION OF THE INVENTION

The present invention seeks to remedy the above-mentioned drawbacks and, for a turbomolecular pump with a flexible mount, it serves to avoid problems of a rotor coming into contact with emergency bearings other than in the event of the magnetic bearings failing or when starting and stopping the pump, and at least to reduce the consequences of any such contact and in particular permanent oscillations, but without that affecting the pumping qualities of the pump having a low level of vibration.

In accordance with the invention, these objects are achieved by a turbomolecular pump with a flexible mount comprising a leaktight casing designed in such a manner as to be capable of being mounted on a structure via a mechanical damper, a vertical support secured to the casing and located therein for supporting a hollow rotor fitted with fins and driven in rotation by an electric motor, at least first and second radial magnetic bearings mounted on the support, at least one axial magnetic bearing mounted on the support, radial detectors for detecting the radial position of the rotor, at least one axial detector for detecting the axial position of the rotor, at least one emergency mechanical bearing, and a control module including at least circuits for powering the electric motor and the axial and radial magnetic bearings, and servo-control circuits for servo-controlling the axial and radial magnetic bearings suitable for correcting any movement of the rotor in translation along three axes xyz in an orthonormal frame of reference and any tilting relative to two tilt axes; the pump being characterized in that the servo-control circuits present low stiffness and include filter means for filtering the signals received by said servo-control circuits to define a limited passband, in that the control module includes a comparator unit for detecting abnormally large movements of the rotor from the signals delivered by the axial and radial detectors, said movements being greater than predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes, and in that the servo-control circuits include a restabilization network and activation means for selectively activating the restabilization network to activate said stabilization network for a predetermined duration Δt after an instant t₁ at which the crossing of at least one of said predetermined thresholds was detected during a movement of the rotor.

Said predetermined duration Δt for selective activation of the restabilization network is settable.

Advantageously, said predetermined duration Δt of selective activation of the restabilization network lies in the range 0.5 seconds (s) to 5 s.

In a possible embodiment, said predetermined duration Δt for selective activation of the restabilization network is fixed.

The predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes are settable.

Advantageously, the predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes lie in the range 25% to 40% of the maximum possible movement of the rotor in the emergency mechanical bearing and, preferably, are of the order of one-third of said maximum movement.

In a particular embodiment, said comparator unit and said means for selectively activating the restabilization network are included in a digital signal processor DSP of the pump including said servo-control circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from the following description of particular embodiments, given as examples, with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal section view of an example of a turbomolecular pump to which the invention is applicable and having active magnetic bearings and a flexible mount;

FIG. 2 is a block diagram showing the main elements of a control module for a turbomolecular pump to which the invention is applicable;

FIG. 3 is a plot showing the position of the rotor relative to a predetermined axis for a traditional turbomolecular pump with a flexible mount, where the initiation of a movement gives rise to a contact with the emergency bearing and permanent oscillations;

FIG. 4 provides plots showing the position of the rotor relative to a predetermined axis in a turbomolecular pump of the invention with a flexible mount, where initiation of a movement is quickly damped; and

FIG. 5 is a flow chart showing certain steps implemented in the servo-control circuits of the control module of a turbomolecular pump of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an example of a turbomolecular pump having active magnetic bearings to which the invention may be applied.

The turbomolecular pump comprises a leaktight outer casing 100 mounted on a base 101 supporting a vertical support 102 having the windings of an electric motor 140 mounted thereon together with radial magnetic bearings 111 and 112 located on either side of the motor 140. The bearings 111, 112 serve to support a central shaft 105 without contact, the top portion of the shaft having a hollow rotor 103 fastened thereto and arranged as a cap relative to the support 102 and carrying fins 104 on its outer face, the motor 140 serving to drive the shaft 105 in rotation. An axial magnetic bearing 115 serves to hold the shaft 105 vertically relative to the support 102. Detectors 113 and 114 for detecting the radial position of the shaft 105 relative to the support 102 are associated with the radial magnetic bearings 111, 112. In the same manner, a detector 116 for detecting the axial position of the shaft 105 relative to the support 102 is associated with the axial magnetic bearing 115. An emergency top bearing 109, e.g. a roller bearing, and an emergency bottom bearing 110 are associated with the magnetic suspension of the shaft 105 in order to support it in the event of the active magnetic suspension failing.

In FIG. 1, the casing 100 is fastened at its top end to a structure 160 via a mechanical decoupling element 150 that acts as a damper, thereby constituting a flexibly-mounted turbomolecular pump capable of damping vibration, e.g. in the event of the structure 160 moving.

In a variant embodiment, the pump could be fastened to a structure not via its top, but via its bottom, with its base 101 being fastened via a mechanical decoupling element analogous to the element 150.

In both cases, a flexible mount is provided, and the pump is not rigidly mounted on the structure 160, such that if a user needs to move the structure 160 while the pump is in operation, for example, then as little vibration as possible is transmitted to the pump.

FIG. 2 shows the main components of a control module for a turbomolecular pump such as that of the present invention.

In FIG. 2, the turbomolecular pump 1 is represented by the rotor 103, the movement detectors 113, 114, and 116, the magnetic bearings 111, 112, and 115, the electric motor 140, and a sensor 170 for sensing the speed of rotation of the rotor 103.

These functional elements of the pump 1 are connected by cables 120 to a control unit 2 that mainly comprises an electrical power supply 12 taking power from an electrical network 16, together with electronic control circuits.

By way of example, the electronic control circuit may comprise a signal processor card 11 including a digital signal processor (DSP) 13, an analog-to-digital converter 14, and a digital-to-analog converter 15.

The radial and axial movement detectors 113 & 114 and 116 are connected to a movement calculation circuit 8 that is itself connected to the analog-to-digital converter 14 in order to provide the main digital signal processor 13 with digital signals representative of the movements of the rotor 103 relative to the casing of the pump.

The digital-to-analog converter 15 receives control signals from the main digital signal processor 13 and delivers analog signals to the driver circuit 9 powering the windings of the radial and axial magnetic bearings 111 & 112 and 115.

The electric motor 140 is itself controlled by the main digital signal processor 13 via a variable-speed drive control circuit 10, the speed of rotation sensor 170 itself being connected to the main digital signal processor 13.

The main digital signal processor 13 is a microprocessor optimized for calculations that serve to perform digital signal processing and in particular to perform control functions and signal extraction functions. The architecture of the microprocessor is optimized to enable it to perform complex calculations quickly, while having easy access to a large number of inputs/outputs. The microprocessor may naturally also be used in combination with other microprocessors that are conventional.

In the context of the present invention, the main digital signal processor 13 defines servo-control of low stiffness and of limited passband (obtained by filtering) so as to obtain a specified low level of vibration.

In order to ensure that in the event of the structure 160 of the pump moving suddenly at an instant t₀, the rolling bearings of either of the emergency bearings 109 and 110 do not come into contact with the rotor 103, e.g. along a radial axis X as shown in FIG. 3, on the rotor 103 moving through a distance X_(m), and in order to prevent unstable permanent oscillating movement 200 continuing thereafter with repeated contacts between the rotor 103 and the emergency bearings 109 and 110, the present invention implements particular means that are explained with reference to FIGS. 4 and 5.

Thus, according to the invention, an abnormally large movement is detected, e.g. due to an external impact on the structure 160 or the pump casing 100, or indeed due to the structure 160 and the pump being moved, and then, for a predetermined duration Δt, the servo-control circuits 13 are switched to a different corrector network in order, where possible, to avoid making contact with an emergency bearing (curve 201 in FIG. 4), or if such contact cannot be avoided (curve 202 in FIG. 4), in order to restabilize servo-control under the best conditions.

FIG. 4 shows the correction process for the example of the rotor moving along a radial axis X as a result of an impact initiated at an instant t₀ on the pump or on its structure 160.

When at a time t₁ the movement of the rotor 103 exceeds a value X_(a), a restabilization corrector network is activated for a duration Δt until a time t₂.

By default, the servo-control conserves characteristics of low stiffness, but in the event of crossings a threshold (here X_(a) along a radial axis X) during movements of the rotor, a stabilization network is activated by the main digital signal processor 13 in order to damp the movements of the rotor 103.

Under such circumstances, if the impact is not too great (curve 201), it is possible to avoid making contact with the emergency bearings, with the curve 201 deflecting before it reaches the contact line corresponding to a movement X_(m).

If the impact is greater and if, in spite of the restabilization network being activated at a time t₁, the rotor 103 nevertheless comes into contact with the emergency bearings (curve 202), then the movements of the rotor 103 are damped quickly and after a predetermined duration Δt, i.e. at a time t₂, the restabilization network is deactivated and the servo-control once more presents its basic characteristics of low stiffness that are optimized for defining a mounting with a minimum of vibration.

The time Δt for which the destabilization network is activated can be determined as a function of the application, and it may be set in advance in predetermined manner. Nevertheless, it is also possible for the activation time Δt to be adapted to the state of restabilization.

Movement is continuously monitored along five axes, and if a threshold is exceeded on one or more axes, then a switchover is made to the restabilization network.

The activation threshold is settable. It is typically one-third of the maximum movement in the emergency bearings, and it may lie in the range 25% to 40% of this maximum movement.

The activation time is also settable. It lies typically in the range 0.5 s to 5 s.

In practice, the servo-control and the settings are adjusted using the signal processor (DSP) 13 of the pump.

FIG. 5 shows an example of a flow chart showing the main steps implemented in the invention in the servo-control circuits of the control module of a turbomolecular pump in order to avoid the damaging consequences due to sudden movements or impacts of the pump or of its structure 160 in the context of a flexibly-mounted turbomolecular pump, where the default adjustment of the servo-control parameters corresponds to low stiffness and a limited passband.

A step 301 consists initially in defining thresholds Sxd, Syd, Szd in the three radial and axial directions of an orthonormal frame of reference xyz associated with the nominal axis of the rotor 103, and thresholds Sxb and Syb corresponding to tilt axes of the rotor 103.

These thresholds Sxd, Syd, Szd, Sxb, Syb preferably lie in the range 25% to 40% of the maximum possible movement of the rotor 103 in the emergency mechanical bearings 109, 110.

In step 302, the position sensors 113, 114, and 116, and the circuits 8, 14, and 13 are used to measure the movements of the rotor 103, either in translation (movements dx, dy, dz) or in tilting (movements bx, by) relative to the five monitored axes.

In step 303, the measured values of the movements dx, dy, dz, bx, by are compared with the respective thresholds Sxd, Syd, Szd, Sxb, Syb predefined in step 301.

In step 304, a test is performed to determine whether at least one of the measured values of the movements dx, dy, dz, bx, or by exceeds the corresponding thresholds Sxd, Syd, Szd, Sxb, or Syb.

If the answer is no, then the method returns to the measuring and monitoring step 302, and the basic characteristics of the servo-control are not modified.

If the answer is yes, then a starting time t₁ is defined in a step 305 together with a correction activation duration Δt.

A step 306 constitutes a step of activating the restabilization network, which network is initiated at time t₁.

A test 307 is performed to determine whether a time t₂ has been reached that corresponds to the time t₁ plus the duration Δt defined for activating the restabilization network.

If the answer is no, then the method remains in step 306 of activating the restabilization network.

If the answer is yes, the method moves onto a step 308 of deactivating the restabilization network, and it then returns to the step 302 of measuring and monitoring the movements of the rotor 103.

Naturally, various modifications and variants of the invention are possible, and for example the control circuits need not include a DSP type digital signal processor, but rather a set of servo-control processors or circuits of other types. 

1. A flexibly-mounted turbomolecular pump comprising a leaktight casing designed in such a manner as to be capable of being mounted on a structure via a mechanical damper, a vertical support secured to the casing and located therein for supporting a hollow rotor fitted with fins and driven in rotation by an electric motor, at least first and second radial magnetic bearings mounted on the support, at least one axial magnetic bearing mounted on the support, radial detectors for detecting the radial position of the rotor, at least one axial detector for detecting the axial position of the rotor, at least one emergency mechanical bearing, and a control module including at least circuits for powering the electric motor and the axial and radial magnetic bearings, and servo-control circuits for servo-controlling the axial and radial magnetic bearings suitable for correcting any movement of the rotor in translation along three axes xyz in an orthonormal frame of reference and any tilting relative to two tilt axes; wherein the servo-control circuits present low stiffness and include filter means for filtering the signals received by said servo-control circuits to define a limited passband, in that the control module includes a comparator unit for detecting abnormally large movements of the rotor from the signals delivered by the axial and radial detectors, said movements being greater than predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes, and wherein the servo-control circuits include a restabilization network and activation means for selectively activating the restabilization network to activate said stabilization network for a predetermined duration Δt after a time t₁ at which the crossing of at least one of said predetermined thresholds was detected during a movement of the rotor.
 2. The pump according to claim 1, wherein said predetermined duration Δt for selective activation of the restabilization network is settable.
 3. The pump according to claim 1, wherein said predetermined duration Δt of selective activation of the restabilization network lies in the range 0.5 s to 5 s.
 4. The pump according to claim 1, wherein said predetermined duration Δt for selective activation of the restabilization network is fixed.
 5. The pump according to claim 1, wherein the predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes are settable.
 6. The pump according to claim 1, wherein the predetermined thresholds Sxd, Syd, Szd, Sxb, Syb relative to said three axes xyz of an orthonormal frame of reference and relative to said two tilt axes lie in the range 25% to 40% of the maximum possible movement of the rotor in the emergency mechanical bearing.
 7. The pump according to claim 1, wherein said comparator unit and said means for selectively activating the restabilization network are included in a digital signal processor DSP of the pump. 